2 Answers
2

Hop utilization - the amount of acid bitterness extracted from hops - is greatly affected by the gravity of the boil, or the concentration of sugars in the boiling wort. The more concentrated the sugars, the less the hops will be utilized. If you are trying to attain a certain level of HBUs in your recipe, you can do one of two things - go with a more diluted boil or use more hops. Hops are expensive, so diluting the boil is a good choice. Once you've decided to dilute the boil, there are two ways to do this - you could go for a full 5 gallon boil or you could do a partial boil, say 2.5 gallons, with less extract in for the majority of the boil time. Given that most extract brewers, myself included, are not set up to do a full 5 gallon boil, adding your LME late is a good way to get the desired level of HBUs without using loads more hops.

Take your favorite recipe and plug it into Beer Calculus. Set it to a 5 gallon average boil. Look at the HBUs. Now change nothing but the boil size. Change it to 2.5 gallons. The HBUs will drop dramatically.
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JackSmithFeb 23 '10 at 18:05

When you brew light-colored beers using extract, you risk what's sometimes called kettle caramelization. Caramelization is actually a misnomer, as the lowest caramelization point for any of the sugars in wort would be above the boiling point of water.

Count the time that it was off heat and returning to boil toward the total time

You can't add all your extract as a late addition because the sugars in malt extract are beneficial to the isomerization of the alpha acids in the bittering hops (possibly by bringing the pH of the wort down?).